Synthesis and Ultrafast Dynamics of a Donor–Acceptor–Donor Molecule Having Optoelectronic Properties

Abstract

The use of push–pull molecules having donor (D) and acceptor (A) parts arranged in different shapes are being widely studied for application in various optoelectronic devices. In this study three covalently linked D–A–D molecules containing three different carbazole derivatives as donor, anthracene as acceptor, and thiophene as spacer have been synthesized and characterized. A detailed stepwise study has been carried out using anthracene, thiophene–anthracene, and carbazole–thiophene–anthracene derivatives so as to indicate the role of each moiety in the molecule. Steady state fluorescence, time-resolved fluorescence, transient absorption, and cyclic voltammetric methods have been employed to understand the intramolecular charge separation (CS) and charge recombination (CR) dynamics in solvents of different polarity. The thermodynamic free-energy obtained by measuring the redox potential and singlet state energy suggested the possibility of electron transfer from the excited singlet state of carbazole moiety to the anthracene entity. Steady state and time-resolved fluorescence studies showed fluorescence quenching of anthracene moiety upon addition of thiophene while highly efficient fluorescence quenching of anthracene moiety was observed on addition of carbazole derivatives. Femtosecond transient absorption studies confirmed the electron transfer to be the mechanism of fluorescence quenching, in which formation and recombination dynamics of electron-transfer products, anthracene radical anion and carbazole radical cation, were analyzed. The rate of charge separation, kCS, was found to be very high for all the three molecules, and it was on the order of 1010–1011 s–1, while the rate of charge recombination, kCR, was observed to be much slower, and it was on the order of 108–109 s–1. The stepwise structure–property relationship leading to the efficient charge separated state established in the systems studied would help in the improved design of optoelectronic materials that use these moieties.